Motor control



06! 27, 1931. 5, w; JONES 1,829,658

' MOTOR CONTROL Filed Jan. 9, 1922 I n ve nbor: BenJnminwJ nes.

f? A" 01 I? R A" l-lisAttorneg- Patented Oct. 27, 1931 UNITED STATES PATENT orrlca BENJAMIN JONES, OF SCEENECTADY, NEW YORK, ASSIGNOR TO GENERAL ELEC- TRIO COMPANY A CORPORATION OF NEW YORK MOTOR CONTROL Application filed January 9, 1922.

My invention relates to the control of electric motors, and it provides improved means whereby electric motors may be started and stopped and generally controlled in a safe,

reliable and eflicient manner.

' or electromagnetic relay which is biased to one position and has two windings, one for governing the opening of the relay and the other for governing the closing thereof. The closing winding is connected across the resistor controlled by the relay so that the relay will not be closed to effect the short circuiting of the resistor until the voltage drop across the resistor has dropped to the predetermined value due to the speeding up of the motor.

While the arrangement described in our said application is entirely operative and practical for customary conditions imposed in accelerating and otherwise controlling an electric motor, the system leaves something to be desired for certain special applications, of which I shall mention one by way of illustration. There are certain applications of motor control in which it is required that the motor accelerating switches shall in all events close successively to accelerate the motor in a plurality of definite accelerating steps, even though the current taken by the motor is in excess of the predetermined value. Such a condition is met with in the control of steel mill auxiliary motors. In the operation of steel mills, and many other applications where the load changes from time to time due to variations in temperature, misali nment, wear, tear, etc.', the control of the e ectric driving motor in accordance with the current taken by the motor has certain disadvantages. For instance, if the load is ex- Serial No. 527,840.

ceedingly heavy on Monday morning due to the friction of the parts being high, it is necessary to adjust the control equipment to take care of the high value because it-is required that the steel mill positively start.

After a few hours operation at this high value of adjustment, the adjustment may be changed for the ordinary conditions encountered. Ordinary steel mill operators and electricians cannot be depended upon to make the proper readjustment of the current limit accelerating value, and experience has shown that these operators will adjust the equipment for a value of motor current at which the mill will be positively started and operated and then let the apparatus continue to run with that adjustment. Time limit control overcomes the objections to current limit control which have been mentioned, but time limit control wastes a great deal of time under some conditions. Thus, if the time limit is adjusted for the most severe conditions, it will be unnecessarily long for the ordinary conditions. fin accurate and reliable timing device is diflicult to obtain, particularly a timing device which will always give the desired timing function. Time limit control also has the very serious objection that unless the timing is made as some function of an operating condition of the motor, a different timing device is necessary for each different application of the motor control.

The present invention'involves the use of the principle of controlling an electric motor in accordance with the rate of change of an electrical condition of the motor circuits, such,.for example, as the rate of change of the motor current or the rate of change of the counter-electromotive force of the motor. Thus, when the motor is first connected to the source of supply, there is a heavy rush of current and the rate of change of the current is comparatively high. If the motor accelerates, the rate of change of. the motor current varies so that the rate of change a proaches zero as the current taken .by t e motor becomes a substantially constant value. Also, when an accelerating resistor is short stant value. When the motor currentbecomes a substantially constant value, an accelerating step will be taken re ardless of the value of the motor current. f the rate of change of the generated or counter-electromotive force of the motor is the controlling function, the arrangement will be such that a control step will be taken when the counter-electromotive force of the motor is a substantially constant value; that is, when the speed of the motor becomes a substantially constant value. Substantiall definite time intervals will occur between t e taking of the various control steps, so that the accelerating steps will not follow each other too quickly, but in all events the control steps will be taken. In case the motor current reaches a dangerously high value, an overload protective device will be relied upon to protect the motor.

In carrying my invention into effect in one'form, I provide a system of control, as shown in Fig. 1, which has certain features of similarit to the system described and claimed in t c said joint application, Serial No. 354,030, filed January 26, 1920, and furthermore embodies the use of an electromagnetic switch or relay, as shown in Fig. 2, for

which said Eugene R. Carichofi and myself filed an application, Serial No. 421,291,'November 2, 1920, a divisional application of the said application, Serial No. 354,030. The electroma netic switch or electromagnetic relay descri ed and claimed in the divisional application, Serial No. 421,291, is connected so that one of the windings is energized to effect the opening of the switch, and the winding which is provided for governing the closing of the switch is energized in accordance with the rate of change of an operating condition of the motor, such as the motor current, the speed of the motor, the counterelectromotive force or the voltage drop across an accelerating resistor, to hold the switch in the open position until the selected condition has substantially ceased to change. In Fig. 3 I have shown a detail of a modification of the arrangement of Fig. 1 in which a series transformer having its primar connected in the motor armature circuit an its secondary connected to the drop-out coils of the accelerating relays is used instead of the shunt transformer arrangement as in Fig. 1, and Fig. 4 is another detail of a modification in which the shunt transformer is connected across the final accelerating resistor.

Referring to Fig. 1, the electric motor M having a series field F is energized from the source of supply through reversing contactors 11. 12, and 14, the air of contactors 11 and 1 1- provided ior connecting the motor to th urce for operation in one direction and the pair of contactors 12 and 13 for connecting the motor to the source for operation in the other direction. Resistance in the armature circuit of the motor for limiting the current taken by the motor at starting is provided. In the embodiment of my invention, shownin Fig. 1, three sections R, R and R of the starting resistance are provided. The resistor R is adapted to be short circuited by means of the electro' magnetic switch or contactor 15, the resistor R by the contactor 16, and the resistor R by means of the contactor 17 These resistor switches are controlled by means of electromagnetic switches or relays which are controlled rcsponsively to the rate of change of the countcr-electromotive force of the motor. The resistance contactors are themselves therefore controlled responsively to the rate of change of the counter-electromotive force of the motor, the arrangement being such that when the motor speeds up at starting and the countcr-electromotive force of the motor has become substantially constant in value, the contactor 15 is energized to close and short circuit the section R whereupon the motor current again rises, the motor accelerates and the counter-electro-motive force increases. When the current again drops or the counter-electro-motive force again becomes a substantially constant value, the contactor 16 closes, followed by the contactor 17 when the motor speeds up and the motor current again becomesconstant. The normally closed relays 18 and 19 are provided for controlling the first resistance contactor 15, the normally closed relay 20 for controlling the contactor 16. and the normally closed relay 21 for controlling the contactor 17. A master controller 22 is provided for controlling the starting, stopping and reversal of the motor, and a line switch 23 is provided for controlling the connection of the motor and its control apparatus to the source of supply.

The relays 18 to 21 inclusive, which are shown very diagrammatically in Fig. 1, are shown in more detail in Fig. 2. Each of these relays is substantially identical in construction, although the windings of the relays 18 and 19 are connected in a difierent manner than are the windings of the relays 20 and 21. For purposes of explanation the relay 20 will be described. This relay comprises a stationary contact 24 and a movable contact 25 for controlling the energization of the coil winding o'f the contactor 16, the movable contact '25 being connected to the pivoted and movable member 26, to which the armature 27 of the upper coil 28 is connected and to which the armature 29 ofthe lower coil 30 is also connected. The member 26 carrying the contact 25 is biased to the unattracted position, that in which the contacts 24 and 25 make engagement, by means of the spring 31. In the embodiment of our invention shown in Fig. 1, the relays are shown as being normally closed but teamed my invention is not necessarily limited thereto, but is likewise applicable to an arrangement in which the relays are normally open. The armature 27 is supported by a bolt 32 having a screw thread whereby the armature may be very nicely adjusted with respect to the core 33 of the electromagnet of which the winding 28 forms a part. A friction member 34 having a certain amount of resiliency is. provided for retaining the armature 27 in the position to which it has been moved. The lower armature 29 is likewise mounted on a bolt- 35 having a screw thread and a resilient friction device 36 for holding the armature in the position to which it-has been adjusted with respect to the core 37 of the electromagnet of which the winding 30 forms a part. It will be observed that the magnetic structure of the upper electromagnet is substantially independent of that of the lower electromagnet and that the two electromagnets may be adjusted substantially independently oi" each other. The winding 28 of the upper electromagnet is connected across the secondary of the potential transformer P. T. and the winding 30 of the lower electro-magnet is connected across the first resistor R The electro-magnets are mounted on an insulating base or support 41.

The relays 18 and 19 are of similar construction to the relay 20, with the exception that the two independent electromagnets of the relay are connected in a particular manner to be presently explained. When the line switch 23 is closed, the two windings of the upper electromagnets of the relays are connected in series across the source of supply 10 and the windings of the lower electromagnets are also connected in series across the source of supply so that these relays are opened. When the master switch 22 is moved to the right so as to energize the contactors 12 and 13 for motor operation in one direction, the

- upper winding of relay 19 is connected across a part of the starting resistance and the upper winding of relay 18 is connected across the remainder of the resistance and the motor armature, so that the relay 19 will close after a redetermined small interval of time. The a justment will be such that the relay will positively close in all cases except where it would be particularly dangerous for the relay to close. The electromagnetic resistance switch 15 is thereby energized to close to initiate the successive closing of the resistance switches as the motor increases in speed or the current taken by the motor becomes a conthe master controller 22 is turned to the left, the line contactors 11 and 14 are energized to close and the upper winding of relay 18 is connected across the same part of the resistance as was previously the upper winding of the relay 19, the upper winding of relay 19 in this instance being connected across the remainder of the resistance and the motor armature. The lower winding of relay 18 is short circuited by contactor 14 and after an interval of time the closing of this relay is given to the 11 per winding. This time interval is desirable to take up the slack in gears and other parts driven by the motor. The relay 19 will close after an interval and effect the closing of the resistance switch 15, but the relay 18 will be held open due to the energization of both of its windings. When the motor has been operating in one direction and the master controller is thrown quickly through the off position to the other operative position, these relays 18 and 19 will prevent the resistance switches 15, 16 and 17 from closing until the current taken by the motor has been decreased to a predetermined value. The motor can therefore be lugged, that is, reversed quickly by throwing the master switch quickly from the one operative position to the other, without serious injury to the motor, since when the master switch is thrown quickly from one operative position 7 to the other, the starting resistance will all be reinserted in the motor armature circuit and this resistance will not be cut out until the motor armature current has dropped to a predetermined value.

As soon as the motor is connected to the source, there will be a voltage drop across the starting resistance and the relays 20 and" the winding 30 of relay 20, and the closing of resistance switch 16 deenergizes the winding of the lower electromagnet of relay 21.

These lower electromagnets are provided for effecting the opening of the rela s and effecting a sequence, and the up or e ectromagnets are provided for governing the closing of the relays. Since the winding 28 of the upper electromagnet is energized from the seeondar of the shunt or potential trans former T. when theresistance contactor 15 closes, the opening coil 30 will be deenergized and the relay will be held in the open position by means of the wmding 28 until the counter-elect-romotive force of the motor' has become substantially constant and the volta e induced in the secondary of the shunt transiormer has become a predetermined minimum value. The relay will then close,

thus energizing the resistancecontactor 16 to close and short circuit the section B of the tactor 16 in a similar manner deenergizes the lower or opening coil of the relay 21 and permits this relay to close when the rate of change of the counter-electromotive force of the motor has become substantially constant.

As thus constructed and arranged, and with the parts in the relative positions shown in Fig. 1, the operation of my invention is as follows:

In order to start the motor, the line switch 23 will first be closed, thereby connecting the upper winding of the relay 18 and the upper winding of the relay 19 in series across the supply circuit, and the lower windings of these. relays in series across the supply circuit. These rela s will therefore open against their bias to t e closed. position. The master switch 22 will then be moved, say for, iiistance to the right to its first operative position, thereby energizing the pair of reversing contaetors 12 and 13 to close so as to connect the motor to the source of, supply with all of the startin resistance in the motor circuit. When the contactors 12 and 13 close, the upper winding of relay 19 will be connected across the portion Rm of the resistor R and the upper winding of the relay 18 willbe connected across the remainder of the resistance and the motor armature. The lower winding of relay 19 will be short circuited by contactor 13 and the lower winding of relay 18 will be connected across the source of supply. The closing of the motor circuit will effect the opening of the normally closed relays 20 and 21, since the lower or opening coil 30 or" the relay 20 is energized responsively to the voltage drop across the resistor R and the upper or closing coil 28 is energized responsively to the rate of chan e of the counter-electroniotive force of the motor, which is responsive to the rate of change of the motor current, The lower or opening coil of the relay 20 is energized responsively to the drop across the resistors R and R and the upper or closing coil is energized responsively to the rate of change of the counter-electromotive force of the motor. The voltage drop across the section Rm will be insuiiicient to hold the relay 19 open so that this relay will close its contacts after an interval of time in accordance with its bias. The setting of this relay will be such that the relay will positively close unless the motor current is a dangerously high value. The resistance contactor 15 will not be closed, however, until the master controller is moved to its second operative position, thereby completing a circuit for the coil windin of the contactor 15 through the contacts the relay 19 and the segments 38 of the master controller. The closing of the contactor 15 will decnergize the lower or opening coil 860i the relay 20, but this relay will not close since it is held in the open position by means of the upper or closing C011 28. The short circuiting of the resistor R will cause the motor 1 current to rise quickly and then fall again as the motor speeds up. This will induce a current in the secondary of the potential motor current has become substantially constant and the motor speed has become substantially constant, the winding 28 will not be energized sufficiently to hold the relay in its open position against its bias to the closed position, so that the relay will automatically close its contacts. The contactor 16 will not be energized to close, however, until the master controller is moved to the third operative position, thereby energizin the coil winding of the contactor 16 throng the contacts 24 and 25 of the relay 20, the segment 39 of the master switch and the contacts of rela 19. The air gap between the armature o the holding electromagnet 28 and the core 33 thereof is such that after the relay closes, the change of current caused by the closing of the contactor 16 will not cause the relay to reopen.

The closing of contactor 16 will deenergize the lower or opening coil of the relay 21, and this relay will be held in the open position by means of the upper or closing coil, so that when the voltage induced in the secondary of the potential transformer P. T. has de creased to the predetermined value, the relay 21 will close its contacts in accordance with its bias. The contactor 17 will not be closed until the master switch has been moved to the fourth operative position so that the coil winding of the contactor 17 may be energized from the source of supply through the contacts of the relay 21, the segment 40 of the master switch and the contactsof relay 19. Because of the air gap between the armature and the core of the holding electromagnet of the relay 21, this relay will not automatically reopen when the contactor 17 closes and the rate of change of the current is thus varied.

If desired, the master switch 22 can be at once thrown directly to the fourth operative position and the resistance contactors will be automatically closed in succession responsively to the rate of change of the counter-electromotive force of the motor. The automatic closing of the resistance contactors can be arrested at any time by moving the controller backward to either the third, second or first operative position of the controller. For instance, if the master controller is first thrown to the fourth position and the automatic cutting out of the resistance has proceeded until the contactor 16 has closed; if the controller is moved backward to the second position, the successive closing of the contactors will be arrested, the contactor 16 will be opened, but the contactor 15 will be maintained closed.

In case the master controller 22 is first thrown to the left, the upper winding of relay 18 will be connected across the part Ra: of the starting resistance R and the upper winding of the relay 19 will be connected across the remainder of the resistance to the motor armature. The reversing contactors 11 and 14 will close and the lower electromagnet of relay 18 will be'short circuited by the contactor 14 and the lower electromagnet of relay 19 will be connected across the supply circuit. After an interval of time, determined by the time taken 'by the short circuited lower electromagnet to release, the 15 relay 18 will close to initiate the automatic cutting out of the starting resistance in the same manner as describedin connection with the relay 19 when thecontroller was moved from its off position toward .the right. It 20 will be understood, of course, that when the master controller is moved to the left, the pair of reversing. contactors 12 and 13 will be deenergized and the pair 11 and 14 will be energized to connect the motor to the source for operation in a different direction than that occasioned by the closing of the contactors 12 and 13. If the ma'ster controller has been thrown to the fourth or full running position to the right and the motor is operating in that direction and it is desired that the motor be quickly reversed, this may be done by throwing the master controller quickly to the fourth or full running position to the left. The pair of contactors 12 and 13 will thereupon be opened, the resistance contactors will all be automatically opened, I the relay 19 will be opened, relay 18 is already open, and the pair of reversing contactors 11 and 14 will be closed. The motor will then be connected tothe source for operation in the reverse direction with all of the starting resistance included in the motor circuit. This is called plugging the motor, and it will cause no damaging effects on the motor since all of the starting resistance has been included in the motor armature circuit so as to cut down the rush of current. When the motor is plugged in that manner a heavy rush of current occurs because of'the' 5 fact that the generated potential of the 11101 tor is assisting the line potential. The relay 18 will be held open temporarily by the upper electromagnet due to the voltage drop across Rm. When the generator action of the motor has substantially ceased and current in the motor armature circuit has decreased to a predetermined value, the relay 18 will close "its contacts to initiatethe successive operation of the resistance contactors in the manner previously described. The relay will be set to close when the motor has substantially come to rest.

It will be observed that the magnetic circuits of the relay coils are independent of r each other and that the relays 18 to 21 inclusive are all capable of a wide range of adjustment. The values at which the relays will open and close may be varied independently of each other, and this wide range of permlssible adjustment makes the system suitable for all kinds of service in which it is desirable that the accelerating steps-be positively taken. It will also be noted that the same relaysand coils may be used for all sizes of contactors and may be located in any desirable place, or may be locked up away from unauthorized persons and away from dirt. The contact parts of the relay are vertical and will thus retain a minimumamount of dirt. The relays, when of the normally closed type, are never called on to break a circuit and thus may be made quite simple and inexpensive, since the closing of av circuit is relatively simple as compared to breaking a circuit, especially an inductive circuit. Because of that fact, the movement of the relay contacts may be anything desired, and the pickup and drop-out values of the relay may be adjusted without regard to the distance which separates the contacts when the relay is open. Because of the wide range of permissible adjustment, either one of the relay coils may be connected so as to assist in effecting the opening of the relay or be connected as the coil which governs the'closing of the relay. Since one of the relay coils is deenergized when the preceding resistance contactor closes, there is little tendency to open the relay or to diminish the pressure on the relay contacts due to the change in current when the resistance is cut out.

A further important advantage obtained by the use of the particular form of relay is that a time limit characteristic in cutting out the starting resistance may be had. Thus, when the coil 30 of relay 20 is short circuited' by the closing of the resistor switch 15, the coil is not instantly deenergized because of its inductive effect, and the armature 29 will therefore be maintained in the attracted position for an appreciable interval of time. This time interval may be varied by varying the'position of the armature 29 on the threaded bolt 35, since the nearer the armature to the core of the electro-magnet, the longer the armature will be held by the magnetization after the coil has been deenergized. A. time interval may also be had due to the short circuiting of the lower coils of therelays 18 and 19.

When a shunt transformer is used as in Fig. 1, the accelerating contactors are closed in accordance with a function of the rate of change of the motor current and with a time function. If the transformer has inductance (L) and resistance (R) factors so proportioned that the ratio L/R is large, then the transformer will function more as a timing device than as a rate of change device. If the ratio L/R is small, then the transformer will have to be larger, but it will function more as a rate of change device. The impressed voltage on the primary of the transformer P. T. will increase as a function of the speed of the motor and the current induced in the secondary of the transformer will be in one direction, approaching zero only when the motor speed has ceased to increase. This produces a result which is desirable for a motor control arrangement, and the L/R factor may he used as best suited to the work to be performed without being otherwise restricted.

In the arrangement shown in Fig. 3, the transformer C. T. is of the series type and is connected in series with the motor armature. The secondary of this transformer is connected to energize the holding coils of the relays 20 and 21 in the same manner as the secondar of the shunt transformer P. T. of Fig. 1. t is considered that those skilled in the art will understand the complete control from the suggestion given in Fig. 3, since the sole difference consists in connecting the primary of the transformer in series with the motor armature instead of in shunt therewith. In this arrangement the control of the resistors is a combined rate of change of the motor current function and a time function with the rate of change function as the predominating factor. Both rate of change and time functions are desirable for a practical control arrangement. The shunt transformer arrangement of Fig. 1 has the advantage over the current transformer arrangement as in Fig. 3 in that the transformer is more readily susceptible of a practical design to obtain the desired relation between the rate of change and the time functions.

In the arrangement of Fig. 4, which like Fig. 3 is merely a detail showing how the arrangement of Fig. 1 may be modified, the primary of the shunt transformer P. T. is connected across the final starting resistor R The transformer is therefore energized responsively to the rate of change of the voltage drop across this resistor. This arrangement has certain of the advantages of the arrangement of Fig. 1, and in additionthe advantage that the transformer is deenergized when the resistor contactor 17 closes. It furthermore has the advanta e that the primary of the transformer may Ee connected across more or less of the starting resistors or be connected across only a portion of the resistor R This permits of readily adjusting the arrangement to obtain the current taken by the motor in starting first increases very rapidl and then decreases more slowly. In t e arrangements of Figs. 3 and 4, if the excitation of the respective transformers exactly follows this change, the induced current in the secondary will be in one direction during the increasing period and in the opposite direction during the decreasing period. But since there is a lag due to the relationship of L/R, the induced current may go through Zero at a critical period and let the next successive resistor contactor close before it should. For this reason, if the transformer is connected as shown in either Fig. 3 or Fig. 4, the transformer should function more as a timing device than as a rate of change device to give the desired positive action of the relays 20 and 21.

In accordance with the provisions of the patent statutes, I have described the principle of operation of myinvention, together with the apparatus which I now consider to represent the best. embodiment thereof; but I desire to have it understood that the apparatus shown is only illustrative, and that the invention can be carried out by other means. V

Nhat I claim as new and desire to secure by Letters Patent of the United States, is,-

1. A control system for electric motors comprising a resistor, an electromagnetic switch having a switch member normally in one position for controlling the resistor, the said switch having two windings which successively control the operation of the switch member, one of which is energized to govern the operation of the switch member to' a second position and the other of which is energized responsively to the rate of change of a motor operating condition to govern the operation of the switch member to the normal position.

2. A control system for electric motors comprising a resistor, an electromagnetic switch having a switch member normally in one position for controlling the resistor, the said switch having two windings which successively control the operation of the switch member, one of which is energized to govern the operation of the switch member to a second position and the other of which is energized while a condition of the motor circult is changing to hold the switch member in the second position and to then'release the switch member to return to the, normal position when the said condition of the motor circuit'h'as substantially ceased changing.

3. A control system for electric motors comprising a resistor, a normally closed electromagnetic switch for-controlling the re-- lit rate of change of a motor. operating conditionand governs the closingof the switch.

4. control system for electric motors comprising a starting resistor, a normally closed electromagnetic switch for controlling the resistor, the said switch having two windings, one of which is energized to effect the opening of the switch and subse uently automatically deenergized, and the ot er of which is energized in accordance with the rate of change of a motor operating condition to hold the switch open until the said motor operating condition has become substantially constant.

5. A control system for electric motors comprising a plurality of resistors, normally closed successively operated electromagnetic switches for controlling the resistors, each of the switches havinga winding for opening the switch effective to hold the switch open until the preceding switch closes,

and means energized responsively to the rate of change of a motor operating condition for holding the switches open until the said motor operating condition has become substantially constant.

6. A'control system for electric motors comprising a plurality of resistors, normally closed successively operated electromagnetic switches for controlling the resistors, each of the switches having a winding for opening the switch which is deenergized by the closing of a preceding switch and a winding for controlling the closing of the switch which is energized in accordance with the rate of change of a motor operating condition.

7 A control system for electric motors comprising a plurality of resistors, normally closed successively operated-electromagnetic switches for controlling the resistors, each of said switches having awinding energized responsively to the voltage drop across a precedingresistor for opening the switch, and a winding energized responsively to the-rate of change of a motor operating condition for holding the switch open until the said motor operating condition has become substantially constant. 7

8. A control system for electric motors comprising a plurality of resistors, a plurality of normally open electromagnetic switches for short circuiting the resistors, normally closed electromagnetic relays for controllingthe operation of the said .electromagnetic switches, each of said relays .having two cooperating windings, one of the windings being connected across one of the resistors to open the relay, and the other winding energized responsively to the rate of change of the motor current to hold the I relay open until the motor current has become substantially constant.

9. A control system for electric motors comprising an electromagnetic switch havmg a switch member biased to one position for controlling the motor, the said switch having two windings, one of which governs the operation of the switch member to another position. a transformer connected to be energized in accordance with conditionsin the motor circuit, and connections whereby the other switch winding is ener gized from the secondary of the said transformer to govern the return of the switch member to its biased position.

10. A control system for electric motors comprising a sourceof supply, motor circuit energized from said source, a switch for controlling the said circuit, the said switch having a switch member and two windings which control.the operation of the switch member, one of said windings being connected to the motor circuit to be energized in accordance with a difference in voltage be tween points in the motor circuit which varies due to a changing operating condition of the motor and which is of a value less than the voltage of the said source, and the other of said windings being inductively related to the motor circuit to regulate the operation of the switch in accordance with the rate of change of a changing condition of the motor circuit.

11. A control system for electric motors comprising a motor circuit, a switch for controlling the acceleration of the motor. the said switch having a switch member and two windings which control the operation of the switch member, one of said windings being connected to be energized responsively to the difference in voltage between points in the motor circuit for holding the switch member in the open position, and the other of said windings being inductively related to the motor circuit to govern the operation of the switch member to the closed position in response to the rate of change of the potential difference between points in the motor circuit.

12. A control system for electric motors closing of the switch.

13. A motor control system comprising a motor circuit including a motor armature,

switch contact mechanism for controlling the said circuit to control the motor, an electroresponsive device having a primary winding connected across the motor armature and a secondary winding inductively related to said primary winding, and means governed by the said secondary winding for governing the operation of said switch contact mechanism in accordance with the rate of change of the generated potential of the motor armature.

14. A control system for electric motors comprising electro-responsive motor controlling means, atransformer having its primary connected across the motor armature, and connections whereby the said means is energized from the secondary of the transformer to control the motor.

15. A control system for electric motors comprising a motor, a resistor, electro-responsive switch mechanism for controlling the resistor, and an electro-responsive device having one winding connected in multiple relation with the motor armature and a second winding inductively related thereto for controlling the said switch mechanism whereby the said switch mechanism is energized I to close and short circuit the resistor when 17. A control system for electric motors comprising an electromagnetic switch having a switch member normally in one position for controlling the motor, the said switch having .two windings which successively control the operation of the switch member, one of which is energized to govern the operation of the switch member to a second position, and means responsive to the rate of change of an operating condition of the motor for controlling the energization of the other of said windings to govern the operation of the switch member to the normal position.

18. A control system for electric motors comprising a resistor for contrlolling" the motor, an electromagnetic switch having a switch member normally in one position for controlling the resistor, the said switch having-two windings which successively control the "operation of the switch member, one oi which is energized to govern the operation of -the switch member to a second position, and

means for supplying a current which is proportional to the rate of change of an electrical condition of the motor circuit to the said other winding to govern the operation of the switch member to the normal position.

19. A control system for electric motors comprising an electro-rcsponsive switch having a switch member biased to one position for controlling the motor, means for operating the said switch member to a second position, and electro-responsive means for exerting on the said switch member a holding force whichvarics responsively to the rate of change-of the difference of potential between points in the motor circuit to hold the switch member in the second position until the potential difference between the said points becomes a substantially constant value.

20. A control system for electric motors comprising a resistor in the motor circuit, an elect-ro-magnetic switch for controlling the said resistor, an electromagnetic relay having a switch member biased to one osition for controlling the energization of t e said electromagnetic switch, the said relay having two windings which successively control the said switch member, and connections whereby the first winding of the said relay is energized to operate the relay to a second position and is subsequently deenergized, and the second winding of the relay is energized in accordance with the rate of change of the motor current to hold the said switch memher in the second position until the motor current has become substantially constant.

21. A control system for electric motors comprising an electro-responsive switch having a switch member biased to one position for controlling the motor, electro-responsive means for operating the said switch member to a second osition, a second electro-respon: sive means or holding the switch member in the second position, an electro-responsive device-connected to be energized in accordance with the rate of change of the difference of potential between points in the motor circuit,

and connections whereby the energizationv of the said second electro-responsive means is controlled b the said device so as to hold the said switc member in the second osition until the change in the difference 0 potential between said points in the motor circuit substantially ceases.

22. A control system for electric motors comprising an electromagnetic switch having a switch member biased to one position for controlling the motor, an electromagnet for operating the switch memberto a second position, a second electromagnet for holding the switch member in the second position, an Y electro-responsive device connected to be energized n accordance with the rate of change of the motor armature voltage, and connections whereby the energization of the said second electromagnet is controlled by the i said device so as to hold the said switchmem- I said second e -comprising an electromagnetic switch having a switch member biased to one position for controlling the motor, an electromagnet for operating the said switch member to the second positlon, a, second electromagnet for holding the switch member in the second position a transformer having its primary connected across points in the armature circuit of the motor and connections whereby the lectromagnet is energized from the secondary of the said transformer to hold the said switch member in the second position until the motor current has, become substantially constant.

24. A control system for electric motors comprising an electromagnetic switch having a switch member biased to one position for controlling the motor, an electromagnet for operating the said switch member to the second positlon, a second electromagnet for holding the switchmember in the second position, a transformer having its primar connected across the motor armature, an connections whereb the said second electromagnet is' energized from the secondary of the said transformer to hold the said switch member in the second position until the counter-electromotive force of the motor has become substantially constant.

25. A control system for electric motors comprising a motor circuit, a resistor in said circuit,'an electromagnetic switch for controlling the resistor, the said switch having two wmdin s, one of which is connected to be energize responsivel to the current taken by the motor to hold t e switch in the open position, and the other of which is connected to the motor circuit to set up a ma etic force which varies responsively to t e rate of change of a motor operating condition to govern the closing of the switch.

26. A control system for electric motors comprising resistance in the motor circuit,

electrom'a etic switch mechanism for controlling t e resistance, means operated responsively to the value of the motor current for holding the said switch mechanism in the open Position and means responsive to the rate 0 change of a motor operatm condition for controlling the closing 0 said switch mechanism.

27. A control system for electric motors,

comprising resistance in the motor circuit,

'electroma etic switch mechanism for controllin e resistance, means responsive to the va ve of the motor current for initiating the operation of the said switch mechanism, and means responsive to the rate ofchan of a motor operating condition for contro ling the subsequent operation of the said switch 28. control system for electric motors, comprising two resistors in the motor circuit, electromagnetic switches for controlling the said resistors, means res onsive to the value of the motor current or controlling one of the said switches, and means responversing switch mechanism,'a reversing controller for controlling the said switches and the said switch mechanism, means responsive to the value of the motor current for controlling one of the said resistor switches, and means responsive to the rate of change of an o crating condition of the motor for controlling the other resistor switch.

30. A reversing control system for electric motors, comprising two resistors in'the motor circuit, electromagnetic switches for controlling the said resistors, electro-responsive reversing switch mechanism, a reversing controller for controlling the said switches and the said switch mechanism, a relay energized responsively to the voltage drop across one of the resistors for controlling the corresponding resistor switch, and a relay energized responsively to the rate of change of a motor 0 erating condition for controlling the other 0 said resistor switches.

31. A control system for electric motors, comprising resistance in the motor circuit, and means operated responsively to the voltage drop across the resistance for controlling the resistance to stop the motor and responsively to the rate of change of a motor operating condition for controlling the resistance to accelerate the motor.

32. A control system for electric motors, comprising resistance in the motor circuit, means operated responsively to the voltage drop across the resistance for controlling the resistance to stop the motor, and means operated responsively to the rate of change of a motor operating condition for controlling the resistance to accelerate the motor.

. 33. A control system for electric motors, comprising a plurality of resistors in the motor circuit, electro-responsive means operated responsively to the voltage drop across one of the resistors for controlling the stopping of the motor, and electro-responsive means operated responsivelytothe rate of change of 'a motor operating condition for controlling the remainder of the resistors to accelerate the motor. I

34. A control system for electric motors, comprising a plurality of resistors in the motor circuit, electromagnetic contactors for controlling the said resistors, electro-responsive reversing switch mechanism for controlling the direction of motor operation, a pair of relays under the control ,of the said reversing switch mechanism for controlling one normally closed switch mechanism for controlling the resistor, means responsive to the value of the motor armature current for operating the said switch mechanism to the open position and holding the same therein, and means responsive to the rate of change of a motor operating condition for influencing the closing of said switch mechanism.

36. A control system for electric motors comprising a resistor in the motor circuit, an electroresponsive switch having a switch member biased to one osition for controlling the resistor, means or operatin the said switch member to a second position, and electroresponsive means separate from said first means inductively related to the motor circuit for causing to be exerted on said switch member a magnetic holding force which varies 'responsively to the rate of change of a changing condition of the motor to influence the return of the switch member to the first position. v

3?. A. control system for electric motors comprising a resistor, a normally closed electromagnetic switch for controlling the resistor, the said switch havin two windings, one oi which is connecte to be energized in accordance with the value of the motor armature current and governs the opening of the switch and the other of which is connected to be energized in accordance with the rate of change of a motor operating condition and influences the closing of the switch.

38. A control system for direct current motors, comprising a resistor in the motor circuit, an electromagnetic switch for controlling the resistor, & relay forcontrolling said switch, the said relay having a winding connected to be energized responsively to the motor current for effecting the initial operation of the relay and a winding energized responsively to t e rate of change of a motor operating condition forinfluencing the subsequent operation of the relay.

39. A control system" for electric motors trolling thekaceeleration of the motor, the

said switch havin a switch member normally in the close position and two windings which influence the operation of the switch member, one of said windin s being connected to the motor circuit to e energized responsivel to the value of the motor current for efiectmg operation of the switch member to the open position and the other of said windings being inductively related to the motor circuit for influencing the closing of the switch member.

40. A control system for direct current motors comprising a motor circuit, means comprising an electroresponsive device for controlling the motor speed, the said device having two cooperating windings, one of which is connected to respond to variations in the value of the current in the motor circuit and the other of said windings being inductively related to the motor circuit, the said device having a movable element which is operated to one position under the influence of both of said windin s and returned to the initial position thereor under predetermined conditions under the control of one of said windings alone.

41. A control system for direct current motors, comprising a resistor arranged to be included in the motor circuit and having a pluralit of sections, a contactor for shunt-' mg eac of said sections, a separate relay for controlling each of said contactors, the said relays being interconnected for o ration in a definite sequence to effect a de ite sequence of operation of said contactors, each of said relays having a switch member and two windings which cooperate to'control the same, one of said windings being connected to be energized to effect the initial operation of the switch member and the other of said windings being connected to respond to the rate of change of a motor operating condition to influence the subsequent operation of the switch member.

42. A control system for electric motors comprising a resistor, switch mechanism for controlling said resistor, electroresponsive means for operating said switch mechanism from a first position to a second position,

and electroresponsive means separate from said first means for regulating the supply of current to operate the said switch mechanism to the first position in accordance with the rate of change of the potential difference between points of the motor circuit and a time function. w

43. A control system for electric motors comprising a resistor, switch mechanism for controlling the resistor, means responsive to thevalue of the motor current for opening said switch mechanism, and electroresponsive -means separate from said first means recomprising'a motor circuit, a switch for con sponsive to the potential difierence between points in th'egrnotor circuit forcontrolling III BENJAMIN W. J ONES. 

